Cathode ray tube comprising a non-rotationally symmetrical element

ABSTRACT

In a cathode ray tube comprising a non-rotationally symmetrical electron-optical element, the image ratio of the main lens is adapted to the degree of astigmatism of the electron beam such that an electron spot having a predetermined axial ratio is produced on a target. Such a cathode ray tube is notably an index colour television tube.

United States Patent [1 1 Scheele Apr. 29, 1975 CATHODE RAY TUBECOMPRISING A NON-ROTATIONALLY SYMMETRICAL ELEMENT [75] lnventor: EdialFrancois Scheele. Emmusingel.

Eindhoven. Netherlands [73] Assignee: U.S. Philips Corporation, New

York. NY.

[22] Filed: Mar. 12, 1973 [2|] Appl. No.: 340,051

[30] Foreign Application Priority Data Mar. 24, 1972 Netherlands 7203931[52] U.S. Cl. 313/453 [5 1] Int. Cl. H0lj 29/56 [58] Field of Search313/86, 453

[56] References Cited UNITED STATES PATENTS 2.840.754 6/l958 Linder etul. 313/86 2.884.559 4/1959 Cooper et al. 3 l 3/86 3.524.094 8/1970Husker et al 3 l 3/86 3.579.010 5/l97l Jones 313/453 PrimaryE.\'aminerVincent P. McGraw Assistant Examiner-Richard A. RosenbergerAttorney, Agent, or Firm-Frank R. Trifari; George B. Berka [57] ABSTRACTIn a cathode ray tube comprising a non-rotationally symmetricalelectron-optical element, the image ratio of the main lens is adapted tothe degree of astigmatism of the electron beam such that an electronspot having a predetermined axial ratio is produced on a target. Such acathode ray tube is notably an index colour television tube.

9 Claims, 5 Drawing Figures CATI-IODE RAY TUBE COMPRISING ANON-ROTATIONALLY SYMMETRICAL ELEMENT The invention relates to a cathoderay tube, comprising an image screen and an electron gun which isprovided with a cathode, a control grid, an acceleration anode, anon-rotationally symmetrical electron-optical element, and a main lensfor the formation of a target of an electron beam to be emitted by thecathode on the image plane.

A cathode ray tube of this kind is known, for example,from US. Pat. No.2,058,482. An electron gun described therein comprises a control gridhaving a nonrotationally symmetrical aperture. Thereby, an electron beamto be emitted by a cathode is deformed into a non-rotationallysymmetrical beam. Requirements are imposed as regards the relationshipbetween the distance from the cathode to the control grid and thelargest transverse dimension ofthe aperture in the control grid.Theseconditions must be satisfied so as to enable to realization of anelectron beam of sufficient current intensity in the case of electrongunscomprising an acceleration anode having a comparatively largeaperture.

The invention has for its object to'provide a-cathode ray tube in whichan accurately defined target having a selected axial ratio can befocussed on the image screen by means of a non-rotationally symmetricalelectron-optical element in the electron gun. To this end. a cathode raytube of the kind set forth according to the invention is characterizedin that the nonrotationally symmetrical electron-optical elementproduces an emissive cathode surface having an oval contour and anastigmatic electron beam, the position of the main lens being adapted tothe degree of astigmatism of the beam such that an image of the electronbeam in the image plane constitutes an accurately defined target havinga predetermined axial ratio.

In a cathode ray tube according to the invention, a target having acomparatively small width can be realized by a suitable choicelof thesaid quantities. The length of the target can thenbe exactly maintainedwithin a predetermined value. The position of the main lens in a cathoderay tube is determined by other conditions in most cases. For example,the distance from the main lens to the image screen is usuallydetermined by the type of tube. Because a maximum spot width may not beexceeded, the magnification factor of the main lens will have to remainbelow a given value. The position of the main lens between an objectpoint of the electron beam and the image screen is determined by thegiven distance between the main lens and the image screen and an imposedmagnification factor. By imparting a degree of astigmatism to the beamwhich corresponds to this position of the main lens, according to theinvention a target can be realized which satisfies the requirementsimposed. In particular, in a preferred embodiment according to theinvention an oval target can be realized in an index colour tube bymeans of which a pure colour image of adequate resolution can be formed.The structure of the image screen in a tube of this kind allows an axialratio of approximately for the target. By choosing the grid aperture ofthe electron gun as the non-rotationally symmetrical element, thecathode load and the space charge in the electron beam are reduced withrespect to a corresponding rotationally symmetrical electron beam.

Some preferred embodiments of cathode ray tubes according to theinvention will be described in detail hereinafter with reference to thedrawings. In the drawmgs:

FIG. 1 is a diagrammatic sectional view of an electron gun which issuitable for a cathode ray tube according to the invention,

FIG. 2 shows a preferred embodiment of an assembly of a grid aperture, afirst anode aperture and a diaphragm of a cathode. ray tube according tothe invention,

FIG. 3 is a diagrammatic view of a beam path of the electron beam,measured in two symmetry planes of the electron gun,

FIG. 4 shows a graph in which a measure for the astigmatism of theelectron beam is given as a function of the axial ratio of the gridaperture, and

FIG. 5 shows sectional views of an index colour tube according to thisinvention.

An electron gun as shown in FIG. 1 comprises a cathode l, a control grid2, a first anode 3, a high voltage anode 4, a main lens electrode 5, anda second high voltage anode 6. In practical electron guns, thecylindrical electrodes are assembled to form one unit by means ofmounting pins 7 and, for example, three glass rods 8. The cathode ispreferably mounted in the control grid tube by means of ceramic rings.The mounting rings fix the distance between the cathode and the controlgrid. A filament 9 is mounted in the grid tube. The cathode consists,for example, of a dispenser cathode having a plate 10 of porous materialsuch as sintered tungsten. An electron gun of this kind can have thefollowing dimensions. Distance between the cathode and the limitation ofthe control grid facing the cathode microns. Thickness of the controlgrid 75 microns. Distance between the control grid and the first anodemicrons. Thickness of the first anode 150 microns. Distance between thefirst anode and the first high voltage anode 4 mm. Distances between thehigh voltage anodes and the main lens anode 2.5 mm. Length of the firsthigh voltage anode 38 mm. Length of the main lens anode 30 mm. Length ofthe second high voltage anode 15 mm. Inner diameter of the anode sleeves20 mm. On its side facing the control grid, the first high voltage anodeis tapered off to 4.5 mm. However, for the invention it is irrelevantwhether the gun is a tetrode gun as described or a triode gun in whichis the first anode is omitted. The main lens can also be formed by anelectromagnetic lens or an accelerating electrostatic lens instead ofthe described unipotential lens.

In a preferred embodiment, the control grid has an aperture 11 of theshape shown in FIG. 2, to be referred to hereinafter as a diamond shape.A diamond shape of this kind is composed of a central rectangle orsquare 12, in this case a square having a side of 250 microns, adjoinedby two triangles 13 which are proportioned such that the overall lengthof the control grid aperture is 1250 microns. The first anode 3comprises an aperture 14 which again has the shape of a diamond, theside of a corresponding square 15 having a length of 450 microns.Including the adjacent triangles 16, the overall length of the firstanode aperture is 1350 microns. FIG. 2 also shows a diaphragm aperture17. In this case the diaphragm aperture is circular and has a diameterof 1,000 microns. A diaphragm plate 18 in which the aperture 17 issituated is arranged in the first high voltage anode 4 as is shown inFIG. 1. The distance between the control grid and the diaphragm amountsto, for example mm. The position of the diaphragm in the anode sleeve isdetermined notably in that the diaphragm must be situated in aunipotential region. Lens-action on the diaphragm is thus prevented. Alens-action at this area would cause high spheric aberration because theelectron beam fills the diaphragm substantially or even completely. Thedimensions of the diaphragm aperture, which may also be elongated, canbe adapted to the optimum position in the anode sleeve. The diaphragmdetermines a maximum value of the transverse dimension of the beam inthe main lens and intercepts stray radiation, for example, caused bygrid emission. In an active gun, the high voltage anodes carry a voltageof, for example. KV, the main lens anode carries approximately 7.5 KVand the first anode carries approximately 500 V. On the basis'ofmeasurements and calculations it can be demonstrated that the opticalproperties of the lens can be adequately described by means of a singlemain plane 19 in front of the main lens and situated in the centre ofthe electrode 5.

A broken line 20 in FIG. 2 denotes a contour of an emissive surface. Theemissive surface of all grid apertures described here has anapproximately elliptical limitation. The major axis of the emissivesurface, to be measured along the line I-'-I in FIG. 2, is directedalong the major axis of the grid aperture, and the minor axis, to bemeasured along the line IIII, is perpendicular thereto. The electronbeam comprises two symmetry planes which are given by the said lines I]and II-II and the optical axis of the system. In normal circumstancesthe target on the image screen has the same direction as the emissivesurface. Hereinafter, the beam dimensions measured in the symmetry planethrough 1-1 will be referred to as major axis, and those situated in thesymmetry plane through IIII as minor axis. The ratio of the axes thusmeasured will be referred to as the ellipticity e, even if the axes aremeasured at a location where the beam section is not an ellipse. Theaxial ratio of the various grid-anode apertures will also be denoted bye. By means of the main lens, an object 21, situated at a distance p infront of the main plane 19, is imaged in a plane 22 which is situated ata distance q behind the main plane 19. In FIG. 3 this image is denotedby a construction beam 23 for an electron beam. The object 21 is in thiscase a cross-over of the electron beam, viewed in the symmetry planethrough the minor axis. In fact, in this case the virtual object ismeant, that is to say the object which is found by linearly extendingthe defining beams of the electron beam in the unipotential region intothe object space as far as the intersection with an optical axis 24 inFIG. 3. In the described gun, J has a value of 60 mm and, if the gun isused in a 90 21 inch index colour tube, q has a value of 400 mm. As anadditional condition it is assumed, by way of example, that the electronbeam extends parallel to the optical axis behind the main plane in thesymmetry plane through the major axis. The reference 25 in FIG. 3denotes a construction beam for the major axis. Consequently, in thisFigure the part of the drawing above the optical axis is situated in thesymmetry plane through the minor axis, and the part of the drawing belowthe optical axis is situated in the symmetry plane through the majoraxis. So the two planes shown in one plane are actually perpendicular toeach other. The imposed requirement can be satisfied by a suitablechoice of the ellipticity e of the emissive cathode surface. Theellipticity thereof can be determined by the shape of the control gridaperture. So as to obtain more insight into this matter, FIG. 4 shows anempirically determined series of curves, in which the degreeofastigmatism of the electron beam, expressed in a length A s (a sagitaldistance 26 as shown in FIG. 3), is given as a function of the axialratio e of the emissive cathode surface, and as a function of the axialratio of the control grid aperture. It was found that the ellipticity ofan elliptical control grid aperture is taken over by the emissivecathode surface. The dependency on current intensity and on the aperturein the first anode or the voltage of the first anode will not be dealtwith in this context. The condition for a parallel, in any casenondiverging, beam in the direction of the major axis can now be simplyderived from FIG. 3. This is given by p z A s(q p).

For p 60 mm and p+q 460 mm, this results in A s s 8 in the describedembodiment. According to FIG. 4, the ellipticity of the emissive surfacemust then be 4. This can be realized by means of a control grid aperturein the shape of an ellipse with e s 4. It is then advantageous toapproximate the maximum permissible value of e as closely as possible.This benefits both the beam diameter and the dimension of the minor axisof the target. An electron gun comprising an ellipse having axes of 350and 1400 microns was in principle found to be satisfactory for a 21 inchtube. In a preferred embodiment, however, a tetrode gun having a gridaperture of a different shape will preferably be used because theadjusting facilities are then increased and less aberration occurs.

If an electron gun of the described kind performs satisfactorily in acathode ray tube, an electron gun for a different type of tube can bedirectly derived therefrom. For example, in 1 10 23 inch index colourtube, in which the distance between the main plane of the main lens andthe screen q 315 mm, a magnification factor of again maximumapproximately 6.5 results in an object distance p 50 mm. It follows fromthe formula p As (q-l-p) that A s s 6. In accordance with FIG. 4, thiscorresponds to a value of 3.5 of the ellipticity of the emissivesurface. This can be realized by means of a grid aperture in the form ofan ellipse having this ellipticity. Measurements have shown thataberrations occur when the target is formed, and that these aberrationscan be reduced by using grid apertures having the shape of a diamond ora shape which is limited by two arcs of a circle, to be referred tohereinafter as a circle peak. The emissive surface is again an ellipse,but its ellipticity is smaller than the axial ratio of the gridaperture. This is denoted in FIG. 4 which shows a curve b for a circlepeak shape and a curve 0 for a diamond shape in addition to a curve afor an ellipse. As result, the advantage of a comparatively long gridaperture is coupled to a small value of A s. The emissive surface can becalculated for any configuration and potential of the control grid andany further electrodes by means of potential calculations for thecathode surface.

A practical advantage of the diamond shape over the ellipse or thecircle peak is that the apertures of the control grid and of the firstanode can be very accurately arranged with respect to each-other when acathode gun is assembled, as is shown in the FIGS. 1 and 2. A slantedorientation cannot only be more readily detected visually in the case oflinear limitations, but a jig can also be used with more accuracy forassembly. The apertures are'provided in the various plates. for example,by spark erosion. ln the case of linear limitations. a higher accuracycan then again be obtained. The grid aperture and' the first anodeaperture can also be orientated to be parallel. This can be used. forexample, in cathode raytub'esin which a line focus is desired such asthe target of an electron beam generating X-rays in an X-ray tubecomprising line focus.

The circumstancesbeing the same, an electron beam can generally befocussed to a smaller target at its length is permitted to be longer.This is due to the fact that the influence of the space charge on theformation ofthe target is reduced because the electron flow isdistributed over a larger (elongated) surface. By using an oval emissivesurface, this gain not only occurs between the main plane and the imagescreen, i.e. mainly near the image screen, but also at the formation ofthe crossover which is now linear instead ofcircular. By suitableproportioning, to be established at a given astigmatism, imaging andcurrent intensity by calculations, a compensating action can be obtainedbetween the space charge effect and the effect of spheric aberration ofthe main lens on the electron beam. As a result, a target can berealized which is narrower than would be possible on the basis of thetwo quantities individually which increase the relevant axis of thetarget.

A preferred embodiment according to the invention in the form of a 90 11 inch index colour tube will be described hereinafter with reference toFIG. 5. The figures show a sectional view in the line direction,coinciding with the symmetry plane through the minor axis of an electrongun 31, a sectional view 32 in the image direction, coinciding with thesymmetry plane through the major axis of the electron gun, and asectional view 33 through a diagonal ofa 90 11 inch envelope. The screen34 in the tube accommodates three colour phosphors which are provided inthe form of lines having a width of approximately 100 microns,transverse to the line direction of the television image, i.e. in thedirection of the major axis. Present between the phosphors, denoted byR, G and B for red, green and blue, respectively, are lines 35 of aninert dark material. These black lines preferably have a width which isequal to that of the colour lines and they are provided so as to ensureproper colour purity of the image. Provided on this line pattern is athin aluminum layer 36 which keeps the entire screen at the samepotential and which also serves for light separation. On this aluminumlayer, the metal backing, phosphor lines 37 are provided behind everysecond black line, the phosphor lines consisting of a phosphor having ashortwave luminescence, to be referred to hereinafter as u.v. phosphor.Radiation pulses to be emitted by the u.v. phos-' phor when passing thetarget, are captured by a photomultiplier 39 via a window 38. By meansof starting lines (not shown) for establishing the correct phase, anindex system is realized by means of which the electron gun can becontrolled in a colour-dependent manner. For the writing of thetelevision image, an electromagnetic deflection unit 40 is providedabout the neck of the envelope. In addition to the cathode l, thecontrol grid 2. the first anode 3, the high voltage anodes 4 and 6, andthe main lens electrode 5, the diagrammatically shown electron guncomprises an electrically conductive connection 41 between theelectrodes 4 and 6, and an electrical conductor 42 between electrode 6and an electrically conductive layer (not shown) on the inner wall ofthe envelope. This conductive layer forms one conductor with the layer36. For the control of the electron gun, passage pins 43 are provided towhich the main lens electrode, the first anode, the control grid. thecathode etc. are connected in an electrically conductive manner. Thedistance between the main plane 19 of the main lens and the image screenamounts to 180 mm in this case. Using a magnification factor 6, p 30 mm,q= l mm, and p +q 210 mm. This results in a value of approximately 4.3for A s. An ellipse with e 3 would satisfy this. However, for thepreviously stated reasons preference is given to a diamond shape, inthis case a diamond having an axial ratio of approximately 4.5. Thediamond shape consists of a rectangle of 150 X 200 square microns, thevalue 150 microns relating to the minor axial direction. The 150- micronsides are adjoined by triangles having a height of 225 microns, so thatthe overall length of the aperture amounts to 650 microns. Particularlythe dimension of the triangles is not very critical, provided that theyare sufficiently high. Using such a control grid aperture and an adaptedfirst anode aperture of approximately 250 X 800 square microns with asquare centre part, an electron gun is realized by means of which a goodimage can be formed in the described tube. A l 1 inch tube is thenexceptionally suitable for use in a portable television receiver. Thehigh voltage must then be adjusted to approximately 15 KV. The degree ofastigmatism of the beam can be changed by varying the voltage on thefirst anode, without the current characteristic of the gun beingexcessively changed. This can be advantageous, for example, if adifferent astigmatism is desired in view of errors caused by thedeflection. In principle, the astigmatism can be dynamically controlledby coupling the first anode voltage to the deflection.

For a 1 1023 inch index colour tube, similar considerations lead to anelectron gun having a diamond shape with a minor axis of 300 microns anda major axis of 1300 microns, in which the centre piece has a length of400 microns. The first anode then has the shape of a diamond with asquare of 550 microns and an overall length of 1650 microns. In alldescribed guns, the diaphragm does not intercept more than 5 to 10percent of the beam current at peak current.

What is claimed is:

l. A cathode ray tube comprising an image screen and an electron guncontaining a cathode, a control grid, an acceleration anode, anon-rotationally symmetrical electron-optical element, a main lens forthe formation on an image plane of a target of an electron beam to beemitted by the cathode and an apertured diaphragm plate disposed at saidmain lens, said nonrotationally symmetrical element producing anemissive cathode surface having an ovall contour and an astigmaticelectron beam, the position of the main lens being adapted to the degreeof astigmatism of the beam such that an image of the electron beam inthe image plane constitutes an accurately defined target having apredetermined axial ratio.

2. A cathode ray tube as in claim 1, wherein said nonrotationallysymmetrical electron-optical element consists of a control grid aperturehaving a non-rotationally symmetrical limitation.

BEST AVAILABLE COPY 3. A cathode ray tube as in claim 2. wherein saidgrid aperture has a smaller surface area than an ellipse havingsubstantially the same axial ratio and width.

4. A cathode ray tube as in claim I, wherein said grid aperture has theconfiguration of an ellipse.

S. A cathode ray tube as in claim I, wherein said nonrotationallysymmetrical element is bounded by straight lines.

6. A cathode rat tube as in claim 1. further comprising a secondnon-rotationally symmetrical electronoptical element.

7. A cathode ray tube as in claim 6. wherein said secondnon-rotationally symmetrical element forms the ode ray tube is an indexcolor television tube.

1. A cathode ray tube comprising an image screen and an electron guncontaining a cathode, a control grid, an acceleration anode, anon-rotationally symmetrical electron-optical element, a main lens forthe formation on an image plane of a target of an electron beam to beemitted by the cathode and an apertured diaphragm plate disposed at saidmain lens, said non-rotationally symmetrical element producing anemissive cathode surface having an ovall contour and an astigmaticelectron beam, the position of the main lens being adapted to the degreeof astigmatism of the beam such that an image of the electron beam inthe image plane constitutes an accurately defined target having apredetermined axial ratio.
 2. A cathode ray tube as in claim 1, whereinsaid non-rotationally symmetrical electron-optical element consists of acontrol grid aperture having a non-rotationally symmetrical limitation.3. A cathode ray tube as in claim 2, wherein said grid aperture has asmaller surface area than an ellipse having substantially the same axialratio and width.
 4. A cathode ray tube as in claim 1, wherein said gridaperture has the configuration of an ellipse.
 5. A cathode ray tube asin claim 1, wherein said non-rotationally symmetrical element is boundedby straight lines.
 6. A cathode ray tube as in claim 1, furthercomprising a second non-rotationally symmetrical electron-opticalelement.
 7. A cathode ray tube as in claim 6, wherein said secondnon-rotationally symmetrical element forms the aperture in a first anodeof said electron gun, said two elements being cross-wise arranged withrespect to each other.
 8. A cathode ray tube as in claim 7, wherein bothnon-rotationally symmetrical apertures individually have the shape of adiamond, the length of a rectangular center part of the first saidaperture being at the most equal to the width of a correspondingrectangular center part in the second said aperture.
 9. A cathode raytube as in claim 1, where said cathode ray tube is an index colortelevision tube.